An induction hardening method applied to a steel is commonly carried out in such a manner that a surface layer of the steel is heated by using an induction current having a frequency of 1 to 400 kHz and then cooled so as to form a hard martensite phase on the surface layer. Such a method is a very general-purposive surface quenching technology and therefore applied to produce a member such as a gear, a spindle, a crankshaft and a camshaft, which requires abrasion resistance, high bearing stress and high fatigue strength.
FIG. 25 is a drawing showing typical induction hardening methods for a gear (as described in Iron and Steel Institute of Japan, “Heat Treatment of Steel”, MARUZEN Co. Ltd, (1985.3.1), p 258). From a productive viewpoint, a method (a) for quenching all of the teeth of the gear at once is widely employed. And, as an induction hardening method in which a quench hardened layer is formed by heat inputting along the teeth profile as shown in FIG. 26A and FIG. 26B, a double induction hardening method (as described in the aforesaid reference, P258) and a rapidly heating method by applying a large power in a moment have been studied and utilized.